This shows how to OR gate two 555, when one 555 cycles at a
low frequency a valve turns on an off, the second 555
stretches the ON duration of the pulse with a diode OR gate.
The OR output uses sample and hold to get the stable analog
data from a sensor after the actuator has gone OFF, this
ensures correct reading.
555 is a fundamental Mixed Signal Circuit as it can be made
into a VCO using Pin-5. If you see old exar databooks, you can
see 555 and PLL and Tone decoders all applications compiled in
I feel the Venerable Signetics 555 "Architecture" and
Intersil ICL8038 'CMOS' were inspiration behind early
communication chip designs, Moving from Bakelite Telephones to
Compact Push Button Electronic Phones and more.
Edit the circuit eagle cad file del00014.zip.
Small inductor in the load is to reduce voltage spikes at switch-on. To Reduce noise use a Gated Zero-Crossing Switch.
This circuit could be modified with a Opto-Isolator with Zero Crossing with at least 5KV Isolation. The reason being this circuit can be a safe only if made into a Hermetically Sealed Module like a Dimmer or Thermostat. The knob of the selector switch and the shaft must have 2KV Isolation for product safety.
In Industrial Timers and even Toaster Timers this "Live"
configuration is common. If you are experimenting with
circuits please do it with Opto-couplers for safety. This
applies for youngsters and students too.
While manufacturing a circuit like this, use a Overrated Thyristor and Fuse for better safety and reliability. You can notice a RC Snubber across the Thyristor to soften Spikes and reduce EMI.
Images above are from the Original Signetics 555 Datasheet
20th Apr 2020
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